fossils of Docofossor (left) and Agilodocodon (right), the earliest-known subterranean and arboreal mammals.

What were the lives of the earliest mammals like? For many, what comes to mind is an image of a small, insect-eating creature that scurried about at night, hiding from dinosaurs and eking out a meager existence. After all, intense competition from the terrible lizards must surely have forced our very distant ancestors into very limited ecological niches.

While there’s some truth to this stereotype, more and more evidence suggests that early mammals were actually remarkably diverse. The fossils of two interrelated ancestral mammals, newly discovered in China, now help confirm that the wide-ranging ecological diversity of modern mammals had a precedent more than 160 million years ago.

With claws for climbing and teeth adapted for a tree sap diet, Agilodocodon scansorius is the earliest-known tree-dwelling mammaliaform (long-extinct relatives of modern mammals). The other fossil, Docofossor brachydactylus, is the earliest-known subterranean mammaliaform, possessing multiple adaptations similar to African golden moles such as shovel-like paws. Docofossor also has distinct skeletal features that resemble patterns shaped by genes identified in living mammals, suggesting these genetic mechanisms operated long before the rise of modern mammals.

These discoveries are reported by international teams of scientists from the University of Chicago and Beijing Museum of Natural History in two separate papers published Feb. 13 in Science.

“We consistently find with every new fossil that the earliest mammals were just as diverse in both feeding and locomotor adaptations as modern mammals,” said Zhe-Xi Luo, PhD, professor of organismal biology and anatomy at the University of Chicago and senior author on both papers. “The groundwork for mammalian success today appears to have been laid long ago.”

Stem mammaliaforms (also known as “stem mammals”) are long-extinct relatives to the extant mammals (crown Mammalia). Docodonts are such a lineage of stem mammaliaforms. Their morphologies provide evidence for the ancestral mammalian condition.

A new docodontan mammaliaform from the Middle Jurassic of China has skeletal features for climbing and dental characters indicative of an omnivorous diet that included plant sap. This fossil expands the range of known locomotor adaptations in docodontans to include climbing, in addition to digging and swimming. It further shows that some docodontans had a diet with a substantial herbivorous component, distinctive from the faunivorous diets previously reported in other members of this clade. This reveals a greater ecological diversity in an early mammaliaform clade at a more fundamental taxonomic level not only between major clades as previously thought.

A new Late Jurassic docodontan shows specializations for a subterranean lifestyle. It is similar to extant subterranean golden moles in having reduced digit segments as compared to the ancestral phalangeal pattern of mammaliaforms and extant mammals. The reduction of digit segments can occur in mammals by fusion of the proximal and intermediate phalangeal precursors, a developmental process for which a gene and signaling network have been characterized in mouse and human. Docodontans show a positional shift of thoracolumbar ribs, a developmental variation that is controlled by Hox9 and Myf5 genes in extant mammals. We argue that these morphogenetic mechanisms of modern mammals were operating before the rise of modern mammals, driving the morphological disparity in the earliest mammaliaform diversification.